個(gè)舊老廠鉀質(zhì)煌斑巖礦物學(xué)特征及其鋯石U-Pb和黑云母40Ar-39Ar年齡

曹華文, 裴秋明, 張壽庭, 王長明, 王光凱, 王 亮

(1. 中國地質(zhì)調(diào)查局 成都地質(zhì)調(diào)查中心, 四川 成都 610081; 2. 中國地質(zhì)大學(xué) 地球科學(xué)與資源學(xué)院, 北京 100083; 3. 山東正元建設(shè)工程有限責(zé)任公司, 山東 濟(jì)南 250100)

0 引 言

鉀質(zhì)-超鉀質(zhì)巖漿巖富集輕稀土元素、大離子親石元素、揮發(fā)分、Cl和F等鹵族元素[1–2]。Müller et al.[2]認(rèn)為富鉀巖漿巖主要形成于五種構(gòu)造背景: 大陸弧、后碰撞弧、晚期洋弧、初始洋弧和板內(nèi)弧環(huán)境[3–4]。富鉀巖漿巖具有多種成因觀點(diǎn): (1) 富集地幔的部分熔融, 沒有地殼物質(zhì)的混染[5–6]; (2) 富集地幔的部分熔融, 且經(jīng)歷地殼物質(zhì)同化混染作用[7–8]; (3) 基性巖漿的部分熔融和結(jié)晶分異, 并經(jīng)歷地殼物質(zhì)的混染[9]。目前, 富鉀巖漿巖的起源較廣泛接受的觀點(diǎn)是:早期的俯沖板片交代地幔楔, 形成的富集地幔在板內(nèi)后碰撞環(huán)境下再部分熔融的結(jié)果[3,10–12]。此外, 富鉀巖漿巖還與多種礦產(chǎn)密切相關(guān), 比如 Cu-Au礦[1,13]、Cu-Ni礦[14]和 Sn-W-Mo-Ag-U 礦[15–17]等。

鉀質(zhì)煌斑巖(即鈣堿性煌斑巖)屬富鉀巖漿巖中重要的中-基性巖類?；桶邘r是富集地幔低程度部分熔融的產(chǎn)物[18], 其化學(xué)成分與源區(qū)成分近似, 能夠揭示地幔信息[5,19]。因此, 鉀質(zhì)煌斑巖不僅能提供地球動(dòng)力學(xué)背景的信息, 如洋殼俯沖作用[20–21]、大陸(巖石圈)伸展減薄作用[14,22–25]、克拉通破壞/巖石圈拆沉作用[5,6,26]和大陸裂解與碰撞作用[27–28]等; 也能由其識(shí)別地幔交代富集過程[8,29]和大陸巖石圈地幔組成的信息[30–31]等。

造巖礦物的化學(xué)組成通常保存了其形成時(shí)周圍巖漿介質(zhì)的物理化學(xué)信息[32–35], 是一種直接研究巖漿結(jié)晶演化及成巖溫壓條件的理想對(duì)象[33]。因此,對(duì)巖漿巖進(jìn)行系統(tǒng)的礦物學(xué)研究對(duì)探討個(gè)舊煌斑巖演化具有重要的意義[36–37]。個(gè)舊錫銅多金屬礦床東區(qū)老廠花崗巖巖體周緣發(fā)育不同類型巖脈, 包括煌斑巖脈、花崗斑巖脈、輝長巖脈和細(xì)晶-偉晶巖脈等[38]。老廠煌斑巖脈是個(gè)舊雜巖體的重要組成部分, 能夠反映個(gè)舊地區(qū)深部構(gòu)造-巖漿作用過程, 所以老廠煌斑巖脈的研究能夠?qū)€(gè)舊雜巖體的成因演化提供重要的證據(jù)。前人對(duì)老廠煌斑巖的研究主要集中在元素地球化學(xué)[39]、同位素地球化學(xué)[40]和鋯石 U-Pb年代學(xué)[41]等, 尚缺少對(duì)煌斑巖造巖礦物的系統(tǒng)分析及更加詳細(xì)的年代學(xué)研究。

本文主要以個(gè)舊老廠東區(qū)迎風(fēng)山、羊壩底、仙人坡和干坡坡煌斑巖脈為研究對(duì)象, 開展輝石、黑云母和長石等的電子探針礦物組成分析、鋯石TIMS U-Pb年齡和黑云母40Ar-39Ar年代學(xué)研究。在前人研究的基礎(chǔ)上, 綜合對(duì)比區(qū)域內(nèi)巖漿巖活動(dòng),揭示區(qū)內(nèi)煌斑巖脈的成因及其與區(qū)內(nèi) Sn-Cu多金屬礦的關(guān)系, 并進(jìn)一步探討煌斑巖形成的地球動(dòng)力學(xué)背景等。

1 地質(zhì)概況

個(gè)舊是世界級(jí)規(guī)模的超大型錫銅多金屬礦集區(qū), 其有色金屬總儲(chǔ)量超過1000萬t, Sn資源量超過 300萬 t[42–44], 是我國重要的 Sn、Cu、Pb和 Zn多金屬資源儲(chǔ)備基地[45]。個(gè)舊礦區(qū)位于揚(yáng)子板塊西緣[46], 即義敦島弧、思茅地塊和華夏板塊的交匯部位(圖 1a)。早新元古代(950～820 Ma), 揚(yáng)子板塊東緣與華夏板塊碰撞[50]; 晚二疊世到早三疊世(260～250 Ma), 揚(yáng)子板塊西緣發(fā)育地幔柱, 峨眉山玄武巖噴發(fā); 晚三疊世(約 230 Ma[51–52])古特提斯洋封閉, 揚(yáng)子板塊西緣與思茅地塊拼合; 中生代侏羅紀(jì)(200 Ma)至今, 研究區(qū)進(jìn)入板內(nèi)演化階段。

個(gè)舊礦區(qū)內(nèi)斷裂和褶皺發(fā)育, 南北向個(gè)舊斷裂將礦區(qū)劃分為東、西兩區(qū), 錫多金屬礦床(點(diǎn))多位于個(gè)舊東區(qū)。北北東向的五子山復(fù)式背斜及東西向的5條壓扭性大斷裂(圖1b)將東區(qū)礦帶自北向南分為馬拉格、松樹腳、高松、老廠和卡房五個(gè)礦田[49](圖 1b)。

礦區(qū)出露的地層主要為三疊系碳酸鹽巖、碎屑巖及基性火山巖(244.4 Ma[42])等, 厚達(dá) 5000余 m;主要由三疊系上統(tǒng)火把沖組(T3h)、鳥格組(T3n), 中統(tǒng)法郎組(T2f)、個(gè)舊組(T2g)和三疊系下統(tǒng)永寧鎮(zhèn)組(T1y)、飛仙關(guān)組(T1f)組成(圖 1a)。區(qū)內(nèi)巖漿活動(dòng)十分強(qiáng)烈, 是該區(qū)形成大型、超大型礦床的重要條件之一, 其中以燕山期中酸性巖漿侵入巖(85～78 Ma[44])分布最為廣泛(圖1b)。

煌斑巖脈主要在個(gè)舊東區(qū)老廠礦田東部出露(圖1c), 圍巖地層為中三疊統(tǒng)個(gè)舊組碳酸鹽巖地層。目前地表見有四條煌斑巖脈, 分別出露于迎風(fēng)山、羊壩底、仙人坡和干坡坡。巖脈主要受北東向和東西向斷裂控制, 沿走向呈斜列式分布, 傾向變化較大(320°～360°), 傾角 50°～80°。

2 煌斑巖巖相學(xué)和地球化學(xué)特征

圖1 個(gè)舊地區(qū)地質(zhì)簡圖及煌斑巖脈分布圖Fig.1 Sketch map showing the geology and distribution of lamprophyres in the Gejiu area

圖2 個(gè)舊煌斑巖顯微鏡下礦物照片(正交偏光)Fig.2 Micro-textures of minerals from lamprophyres in the Gejiu area (Cross-polarized light)

老廠煌斑巖呈深灰綠色, 變余煌斑結(jié)構(gòu), 塊狀構(gòu)造。巖石由斑晶、微晶(基質(zhì))和少量杏仁體組成。研究區(qū)煌斑巖礦物含量的特征如下, 斑晶包括蝕變單斜輝石(＜ 5%)、橄欖石(＜ 5%)、黑云母(＜ 5%)、石英(約 10%)和斜長石(約 5%), 呈自形半自形晶, 粒徑0.5～8.6 mm(圖2)。橄欖石幾乎全為假象晶, 蛇紋石化嚴(yán)重, 偶見包橄結(jié)構(gòu)。單斜輝石呈無色—淺黃綠色柱狀, 有的核部被綠泥石、黑云母交代。黑云母含Ti較高, 呈紅褐色片狀。石英和斜長石具熔蝕現(xiàn)象, 斜長石為中長石, 具聚片雙晶結(jié)構(gòu), 部分具麻點(diǎn)狀邊環(huán)及碳酸鹽化。研究區(qū)煌斑巖中的石英斑晶與哀牢山縫合帶北衙地區(qū)[53]的特征類似, 可能屬于基性巖漿與中酸性巖漿混合的標(biāo)志, 或者是富含揮發(fā)分的煌斑巖漿快速上升侵位的結(jié)果。微晶由鉀長石(30%～35%)、蝕變單斜輝石(約 20%)、黑云母(15%～20%)和少量石英(＜ 5%)組成, 粒徑多小于0.3 mm。鉀長石呈他形—半自形板狀微晶, 單斜輝石、黑云母特征及蝕變現(xiàn)象同斑晶, 石英呈他形粒狀充填上述礦物粒間。少量杏仁體呈不規(guī)則形狀, 被綠泥石、碳酸鹽、石英充填。副礦物主要有磁鐵礦、磷灰石及鋯石等。據(jù)此礦物學(xué)特征, 研究區(qū)煌斑巖應(yīng)為蝕變橄輝云斜煌斑巖[32,54,55], 礦物成分特征與遼東半島鉀玄質(zhì)云斜煌巖[6]類似。在巖石化學(xué)組成上[39], 老廠煌斑巖為鉀質(zhì)鈣堿性煌斑巖, 屬超鉀質(zhì)巖石[56]系列。

3 樣品分析方法

測試樣品選自老廠迎風(fēng)山(LCD01、LCD02)、羊壩底(LCD04、LCD06)、仙人坡(LCD07、LCD08)和干坡坡(LCD13)的新鮮煌斑巖, 采樣位置如圖 1c所示。電子探針(EPMA)礦物學(xué)組成的測試由中國地質(zhì)大學(xué)(北京)電子探針實(shí)驗(yàn)室完成。樣品制備成0.05mm厚的光薄片, 測試儀器為EPMA-1600型電子探針; 測試條件: 加速電壓 15kV, 束流 20nA, 束斑直徑1μm; 分析誤差一般小于1%; 測試礦物為輝石、黑云母、長石、磷灰石和磁鐵礦。

融合DPI設(shè)備應(yīng)對(duì)移動(dòng)通信網(wǎng)絡(luò)的省集中核心網(wǎng)的信令面（S6aa、S1-MME、S110/S11,Gx/RRx、Radius、L2TP、Mw/MMg/Mj/ISC/GGm、Cx/Dx/SSh/Zh等接口）及用戶面（S1-U、S5//8、S2a、Gnn/Gp等接口）所涉及的鏈路進(jìn)行采集。如下圖所示：

煌斑巖樣品鋯石和黑云母單礦物的分選通過全巖粉碎、過篩、手工淘洗、重液分離和實(shí)體顯微鏡檢查等工序完成, 鋯石和黑云母單礦物樣品的純度大于99%。

煌斑巖顆粒鋯石TIMS U-Pb定年由核工業(yè)北京地質(zhì)研究院分析測試研究中心完成。測試儀器為ISOPROBE-T 熱電離質(zhì)譜, 詳細(xì)測試流程和實(shí)驗(yàn)方法參見文獻(xiàn)[57–59], 實(shí)驗(yàn)全流程Pb空白為0.005～0.03 ng,U空白為0.002 ng。

煌斑巖黑云母單礦物樣品與北京房山花崗巖黑云母(ZBH-25)和美國魚谷凝灰?guī)r中的透長石(FCT-01)一起送往中國原子能研究院的原子反應(yīng)堆 B4孔道進(jìn)行快中子照射。照射時(shí)間為8.06 h, 快中子通量為1.86×1017n/cm2。樣品測量在中國地質(zhì)大學(xué)(北京)科學(xué)研究院實(shí)驗(yàn)中心同位素地質(zhì)年代室MM-5400質(zhì)譜儀上完成, ZBH-25標(biāo)準(zhǔn)樣品年齡為(133.2±0.2) Ma,FCT-01標(biāo)準(zhǔn)樣品年齡為(27.8±0.1) Ma。實(shí)驗(yàn)流程詳見王瑜等[60]。Ar-Ar坪年齡譜及等時(shí)線年齡計(jì)算應(yīng)用Isoplot 3.75程序[61]分析, 誤差精度為2σ。Ar-Ar年齡測試的詳細(xì)測試方法及步驟見文獻(xiàn)[62]。

4 分析結(jié)果

4.1 鋯石TIMS U-Pb年齡

個(gè)舊煌斑巖中的鋯石為淺黃色、透明、自形長柱狀、柱狀晶體。鋯石的U-Pb同位素測定及表面年齡計(jì)算結(jié)果列于表 1和圖 3。鉀質(zhì)煌斑巖中的鋯石通常較少[8], 僅挑選出極少量巖漿鋯石。本次實(shí)驗(yàn)對(duì)2顆鋯石進(jìn)行了U-Pb同位素測定。其普通鉛含量較低,經(jīng)對(duì)實(shí)驗(yàn)空白校正后的206Pb/204Pb值較高, 說明其測試精度較高[59]。2個(gè)測點(diǎn)均位于協(xié)和線上, 具有一致的206Pb/238U 表觀年齡, 因此其加權(quán)平均值(87.1±1.3) Ma(MSWD = 0.002)代表鋯石的形成年齡。該年齡與老廠似斑狀花崗巖鋯石((83.3±1.6) Ma[44])和老廠等?；◢弾r鋯石年齡((85.8±0.8) Ma[44])基本一致(圖1b)。

4.2 黑云母40Ar-39Ar年齡

經(jīng)過 13～14個(gè)階段的逐級(jí)加熱后得到研究區(qū) 6個(gè)煌斑巖黑云母斑晶樣品40Ar-39Ar年齡數(shù)據(jù)(表 2),從而獲得樣品的40Ar-39Ar年齡圖譜和40Ar-39Ar反等時(shí)線年齡(圖4)。

迎風(fēng)山(LCD01)黑云母的坪年齡為(71.5±0.4) Ma,反等時(shí)線年齡為(71.8±0.3) Ma (MSWD = 1.17) (圖4a和 4b); 迎風(fēng)山(LCD02)黑云母的坪年齡為(74.2±0.4) Ma, 反 等 時(shí) 線 年 齡 為 (75.5±1.3) Ma(MSWD = 2.3) (圖 4c和 4d)。羊壩底(LCD04)黑云母的坪年齡為(74±0.3) Ma, 反等時(shí)線年齡為(74±1.3) Ma(MSWD = 6.1) (圖4e和4f); 羊壩底(LCD06)黑云母的坪年齡為(73.8±0.4) Ma, 反等時(shí)線年齡為(74.3±1.1) Ma (MSWD = 4.4) (圖 4g 和 4h)。仙人坡(LCD07)黑云母的坪年齡為(70.2±0.4) Ma, 反等時(shí)線年齡為(71.2±0.4) Ma (MSWD = 0.58) (圖4i和4j);仙人坡(LCD08)黑云母的坪年齡為(71.3±0.4) Ma,反等時(shí)線年齡為(73.1±2.2) Ma (MSWD = 7.6) (圖4k和 4l)。

上述6個(gè)樣品的坪年齡和反等時(shí)線年齡在誤差范圍內(nèi)均一致, 且40Ar/36Ar初始比值(圖4)和現(xiàn)代大氣 Ar比值(295.5±0.5)較接近, 說明測試的樣品中不存在過剩的Ar, 并且也無顯著的Ar丟失。指示樣品自結(jié)晶作用以來對(duì)K和Ar保持封閉體系, 受后期地質(zhì)熱事件的擾動(dòng)較弱, 均為可信的地質(zhì)年齡。所以, 老廠煌斑巖形成時(shí)代為 75～70 Ma, 平均年齡為 72.9 Ma, 該年齡與老廠-卡房(老卡)花崗巖體鉀長石40Ar-39Ar年齡近一致(71.6 Ma[63])。

圖3 煌斑巖中鋯石U-Pb年齡協(xié)和圖Fig.3 U-Pb concordia diagram of zircons from lamprophyres

表1 個(gè)舊煌斑巖中鋯石TIMS法U-Pb同位素年齡分析結(jié)果Table 1 Analytical results of zircons from lamprophyres in the Gejiu area

表2 個(gè)舊老廠煌斑巖脈黑云母在加熱過程中的40Ar/39Ar釋放數(shù)據(jù)Table 2 40Ar/39Ar stepwise heating data for biotite from lamprophyres

(續(xù)表 2)

4.3 輝石的礦物學(xué)特征

本次研究采集的樣品中單斜輝石發(fā)生了部分蝕變; 此外, 輝石在結(jié)晶以后易與基質(zhì)中甚至相鄰的礦物發(fā)生元素交換, 這會(huì)導(dǎo)致輝石成分的改變, 故輝石成分變化較大。單斜輝石電子探針分析結(jié)果(表 3)顯示, 煌斑巖中的單斜輝石斑晶和微晶成分近一致,總體表現(xiàn)為貧Ti、Al、Na, 富Ca、Fe和Mg特征。

表3 研究區(qū)單斜輝石化學(xué)成分(%)及端元組分計(jì)算Table 3 Chemical composition (%) and end-member proportion of clinopyroxene in the study area

(續(xù)表 3)

(續(xù)表 3)

Ca∶Mg∶∑(Fe+Mn)平均為 43.65∶47.75∶8.6, 屬 Ca-Fe-Mg輝石族(圖 5a); 在輝石 Wo-En-Fs分類圖中[64],主要位于普通輝石范圍內(nèi), 靠近透輝石區(qū)域(圖 5b)。其Ti-Al特征類似于超鉀質(zhì)巖-過渡帶輝石[66](圖5c)。輝石 Mg#值分布于 79.7～100之間, 在單斜輝石SiO2-Al2O3圖解中[67](圖6a), 主要落入亞堿性區(qū), 少部分落入堿性區(qū); 在Si-AlⅣ圖解[68](圖 6b)和 Ti-(Ga+Na)圖解[69]中(圖 6c), 均位于拉斑玄武巖區(qū)。上述特征暗示其母巖漿屬于堿性-亞堿性系列, 拉斑玄武巖漿; 并且,輝石單礦物與全巖成分[39]的巖漿系列判別結(jié)果一致。

圖5 個(gè)舊煌斑巖輝石分類圖解Fig.5 Classification diagrams for clinopyroxenes from lamprophyre veins

圖6 單斜輝石巖漿系列判別圖解Fig.6 Discrimination diagram of the clinopyroxene series in lamprophyre veins

由輝石 Al2O3-Mg#圖解[70](圖 7a)和 FeOt/MgO(輝石)-FeOt/MgO(全巖)圖解[71](圖 7b)可知, 輝石總體是沿平衡結(jié)晶的趨勢(shì), 表明輝石與其寄主巖石達(dá)到了平衡。少部分輝石顆粒 KD(cpx)大于 0.4, 指示可能為早期結(jié)晶的產(chǎn)物。因此, 煌斑巖結(jié)晶時(shí)的巖漿系列同母巖漿系列一致, 這證明巖漿源區(qū)熔融程度低, 且?guī)r漿在演化過程中沒有經(jīng)歷充分的分離結(jié)晶作用。

隨著單斜輝石的結(jié)晶, FeOt與MgO、CaO均呈負(fù)相關(guān)關(guān)系, 表明發(fā)生了Fe2+對(duì)Mg2+和Ca2+的替代;這種結(jié)晶趨勢(shì)[72]表明單斜輝石形成于中高溫環(huán)境(＞ 1000 )℃。應(yīng)用輝石-熔體溫壓計(jì)[73]可以估算巖漿形成的溫度和壓力條件。據(jù)此計(jì)算出的溫度稍高于二輝石溫度計(jì)(圖 5b)估算的結(jié)果。單斜輝石斑晶和微晶形成溫度分別為1219～1282 (℃平均1255 )℃和1159～1215 (℃平均1199 ); ℃壓力分別為0.3～1.1 GPa(平均 0.86 GPa)和 0.1～1.3 GPa (平均 0.62 GPa); 深度估算采用的靜巖壓力梯度為 30.3 MPa/km, 則其對(duì)應(yīng)的結(jié)晶深度分別為 10～50 km (平均 29 km)和4～44 km (平均20 km)。輝石形成壓力和深度變化較大, 從下地殼到中上地殼均有輝石結(jié)晶形成, 而形成溫度較高且穩(wěn)定; 這證明其不是在深部巖漿房中結(jié)晶后再隨巖漿侵入地表時(shí)帶出, 而是隨巖漿快速上升過程中逐步結(jié)晶而成。

在島弧環(huán)境中, 母巖漿富水, 具有較高的氧逸度, 單斜輝石主要采用ⅣMgⅣSi =ⅥFe3+ⅣAl方式替代, 形成 CaFe3+AlSiO6分子, 因此相對(duì)于非造山環(huán)境的單斜輝石, 相對(duì)富Si, 貧Al和Ti, 且Alz/Ti比值遠(yuǎn)遠(yuǎn)大于非造山環(huán)境[74]。老廠煌斑巖樣品從輝石斑晶到輝石基質(zhì), 存在微弱的由透輝石向普通輝石演化的趨勢(shì)(圖5b), 表明存在Fe對(duì)Mg的替代關(guān)系; 這同時(shí)可從輝石FeO與MgO呈負(fù)相關(guān)性得到驗(yàn)證。這種替代關(guān)系證明研究區(qū)煌斑巖漿起源于島弧環(huán)境[75], 巖漿體系為高溫, 中等大小氧逸度的特點(diǎn)。

單斜輝石 Alz-TiO2圖[74]中(圖 8a), 依靠斜率可以判別老廠煌斑巖為與弧有關(guān)的玄武巖。單斜輝石主要氧化物含量百分?jǐn)?shù) F2-F1雙因子圖解[76]中(圖8b), 老廠煌斑巖單斜輝石樣品投入“火山弧+洋底玄武巖區(qū)”, 更靠近火山弧玄武巖區(qū)域。表明其母巖漿的形成與大洋板塊向大陸板塊的消減過程中的洋殼板塊的熔融有關(guān)。

4.4 黑云母的礦物學(xué)特征

黑云母中的 Fe3+和 Fe2+含量采用林文蔚和彭麗君[77]的待定礦物學(xué)式陽離子數(shù)法確定。黑云母的主元素探針分析結(jié)果列于表 4。黑云母中的Fe2+/(Fe2++Mg)比值均一, 說明老廠煌斑巖未遭受后期流體的顯著改造[78]。老廠煌斑巖黑云母斑晶和黑云母微晶具有較明顯的差異, 微晶黑云母更富TiO2、MgO, 相對(duì)貧 FeO。兩者鎂鐵比值(MF)分別平均為 1.02和 1.28; 含鐵系數(shù)(Fe’)分別為 0.52和0.38, 含鎂系數(shù)(Mg’)分別為 0.48和 0.62, Mg#分別為51.1和 64.0。黑云母成分分類圖解中(圖 9a), 斑晶屬鐵質(zhì)黑云母和鎂質(zhì)黑云母; 微晶云母屬鎂質(zhì)黑云母。

圖7 單斜輝石平衡演化判別圖解(底圖分別據(jù)文獻(xiàn)[70]和[71])Fig.7 Discrimination diagram of the evolution for clinopyroxenes from lamprophyre veins

圖8 輝石構(gòu)造背景判別圖解Fig.8 Tectonic environment discrimination diagrams for clinopyroxenes from lamprophyre veins

表4 研究區(qū)黑云母化學(xué)成分組成Table 4 Chemical composition of biotite in the study area

(續(xù)表 4)

(續(xù)表 4)

圖9 煌斑巖黑云母命名、氧逸度和溫度判別圖解Fig.9 Discrimination diagram of the classificatiton, temperature, pressure and oxygen fugacity for biotites from lamprophyre veins

富Mg、高Fe3+/Fe2+比值的黑云母屬于典型的鉀玄質(zhì)巖石系列[2]。與磁鐵礦、鉀長石共生的黑云母成分可以估算巖漿結(jié)晶時(shí)的氧逸度和溫度[80–82]。個(gè)舊煌斑巖黑云母樣品大致落在 Fe3+-Fe2+-Mg圖解中Ni-NiO(NNO)緩沖線上(圖 9b)。結(jié)合其黑云母穩(wěn)定度(100×Fe/(Fe+Mg))投影到黑云母 lgfO2-t圖解(圖9d), 得到黑云母斑晶和微晶其形成時(shí)大致的溫度分別為 780～900 ℃和 880～1020 ℃, 氧逸度 lgf(O2)分別為–11～ –15 和–10～ –13。此方法獲得的黑云母形成溫度與Henry et al.[81]設(shè)計(jì)的“變質(zhì)巖中黑云母Ti溫度計(jì)”投影的結(jié)果(圖 9c)近似, 但稍偏高。黑云母全鋁壓力計(jì)可以估算其結(jié)晶形成時(shí)的壓力和深度[85],個(gè)舊煌斑巖黑云母斑晶和微晶的結(jié)晶壓力分別為33～116 MPa (平均 82 MPa)和 35～135 MPa (89 MPa),其對(duì)應(yīng)的結(jié)晶深度分別為 1.2～4.4 km(平均 3.1 km)和 1.3～5.1 km (平均 3.4 km)。

FeOt/(FeOt+MgO)-MgO物質(zhì)來源判別圖解[83](圖10a)中, 黑云母斑晶和微晶均位于典型殼?；煸春偷湫蜌ぴ捶秶纸缇€附近, 表明其物源以殼源物質(zhì)為主, 微晶黑云母中幔源物質(zhì)混入更多。在MgOFeOt-Al2O3圖解[84](圖 10b)中, 主要屬造山帶鈣堿性巖漿巖系列。綜上所述, 黑云母斑晶和黑云母微晶之間成分、形成溫度和深度均存在較大的差異;微晶更偏幔源, 斑晶更偏殼源。

4.5 長石的礦物學(xué)特征

長石的主元素探針分析結(jié)果列于表 5。老廠煌斑巖長石斑晶為斜長石(An25～40Ab57～69Or3～6), 具有較明顯的環(huán)帶結(jié)構(gòu)和溶蝕結(jié)構(gòu)。長石微晶為堿性長石(An1～6Ab21～38Or59～78)。在長石分類圖上斑晶和微晶分別屬于中長石-更長石和透長石(圖11)。中長石斑晶的SiO2與Al2O3、CaO、An呈良好的負(fù)相關(guān)關(guān)系,而與 Na2O呈正相關(guān)關(guān)系, 表明斜長石遵循巖漿結(jié)晶分異的演化規(guī)律, 分異完全。從中長石斑晶到透長石微晶, Al2O3、CaO、An和Na2O均急劇降低, SiO2和K2O急劇升高。透長石微晶的化學(xué)成分與卡房花崗巖中的堿性長石接近[86]。

5 討 論

5.1 煌斑巖的形成時(shí)代

在鈣堿性基性巖漿中, 輝石、鈦鐵礦和榍石等的結(jié)晶會(huì)優(yōu)先帶走 Zr元素; 即, 在通常情況下, 鈣堿性基性巖漿中Zr不能達(dá)到飽和形成巖漿鋯石[8]。所以, 鈣堿性基性巖漿巖(如鉀質(zhì)煌斑巖)中的鋯石大部分是捕獲鋯石或繼承鋯石[88–91]。程彥博等[40–41]測試的老廠煌斑巖鋯石 U-Pb年齡諧和曲線圖中, 年齡值出現(xiàn)“分堆”現(xiàn)象(圖 12b), 測試結(jié)果離散(MSWD =20[40–41]), 這可能是由于其包含兩類鋯石引起的。

圖10 煌斑巖構(gòu)造背景與物質(zhì)來源圖解Fig.10 Tectonic environment discrimination diagrams for biotites from lamprophyre veins

表5 研究區(qū)長石化學(xué)成分(%)及端元組分計(jì)算Table 5 Chemical composition (%) and end-member proportion of feldspar in the study area

一類是捕獲鋯石, 即老廠似斑狀花崗巖體的鋯石(約82 Ma), 代表了煌斑巖上侵過程對(duì)老廠花崗巖的捕獲同化。并且程彥博等[40]測試的煌斑巖(鐵鎂質(zhì)巖墻)鋯石 εHf(82 Ma)為–9.5～ –5.1(平均值為–7.1),tDM2年齡平均值為 1300 Ma, 證明鋯石應(yīng)屬殼源巖漿鋯石, 即反映的是捕獲的圍巖(花崗巖)的鋯石特征, 因此該鋯石年齡值不能代表煌斑巖侵位的年齡。另一類為少部分巖漿鋯石, 即煌斑巖巖漿冷凝結(jié)晶時(shí)形成的巖漿鋯石(約73 Ma), 代表煌斑巖的形成時(shí)代。程彥博等[40]測試的第二類鋯石U-Pb年齡與本次黑云母Ar-Ar研究結(jié)果(75～70 Ma, 平均72.9 Ma)一致(圖12), 表明老廠煌斑巖(約73 Ma)晚于個(gè)舊雜巖體巖基(中酸性侵入巖)的主要形成時(shí)代(85～78 Ma[40]), 屬于個(gè)舊雜巖體后基性巖墻群的一部分。

圖11 煌斑巖長石分類圖(底圖據(jù)文獻(xiàn)[86])Fig.11 Classification diagram for feldspars from lamprophyre veins

圖12 個(gè)舊煌斑巖黑云母Ar-Ar年齡、鋯石U-Pb年齡和老廠花崗巖鋯石U-Pb年齡Fig.12 Age distribution of lamprophyre veins and granites in the Gejiu ore district

此外, 個(gè)舊地區(qū)除煌斑巖外的成巖-成礦事件均發(fā)生在77 Ma前, 煌斑巖脈黑云母Ar-Ar反映的73 Ma左右的熱事件只有代表煌斑巖脈的侵位事件。而且, 這也從側(cè)面印證個(gè)舊錫銅多金屬成礦熱液活動(dòng)(95～77 Ma[40,63])應(yīng)與煌斑巖脈的侵位無關(guān),主要受花崗巖漿熱液活動(dòng)(85～78 Ma)影響。

5.2 煌斑巖的地球動(dòng)力學(xué)背景

鉀質(zhì)煌斑巖通常發(fā)育于板塊匯聚和主動(dòng)大陸邊緣[18], 構(gòu)造背景為碰撞后環(huán)境[8]。老廠煌斑巖單斜輝石的起源環(huán)境判別圖解(圖 8)表明其為與火山弧有關(guān)的巖漿源區(qū), 但是個(gè)舊地區(qū)在晚白堊世(85～78 Ma)已經(jīng)是一個(gè)大陸板內(nèi)環(huán)境, 因此單斜輝石反映的島弧成分特征是其母巖漿源區(qū)繼承了先前的島弧(如特提斯洋閉合時(shí)期)地幔源區(qū)的特征。中國東部在晚白堊世時(shí)期(89～87 Ma)受東側(cè)太平洋板塊加速俯沖的影響, 擠壓應(yīng)力和壓縮達(dá)到峰值[92], 個(gè)舊雜巖體(85～75 Ma)的形成稍晚于擠壓應(yīng)力峰值, 形成于擠壓應(yīng)力釋放后的拉伸背景。

個(gè)舊煌斑巖形成時(shí)代稍晚于右江盆地東緣(廣西河池)的大化隴長煌斑巖(89 Ma[22])和羅城壘洞煌斑巖(約100 Ma[90]) (圖1a和表6)。隴長煌斑巖和壘洞煌斑巖均起源于 EM2型巖石圈地幔, 隴長煌斑巖Nd模式年齡較老(2.1～2.2 Ga)。程彥博[40]研究發(fā)現(xiàn)老廠煌斑巖的(87Sr/86Sr)i為 0.7099～0.7114 (平均0.7108), εNd(t)值為–8.8～ –6.8 (平均–8.1), 認(rèn)為其可能來源于富集地幔的部分熔融, 并有顯著的地殼混染[40]。個(gè)舊煌斑巖與隴長煌斑巖在巖石地球化學(xué)和同位素地球化學(xué)方面一致, 表明個(gè)舊地區(qū)與廣西隴長地區(qū)在晚白堊世時(shí)期皆處于伸展背景。EM2型巖石圈富集地幔來源于次大陸巖石圈地幔的交代作用,與古老洋殼俯沖、脫水、交代有關(guān)[18]。白堊紀(jì)時(shí)期,華南地區(qū)的陸內(nèi)伸展作用促使早期(元古宙或晚古生代)俯沖作用形成的交代富集地幔(EM2型)發(fā)生部分熔融形成隴長、壘洞和個(gè)舊老廠煌斑巖漿; 區(qū)域上的殼幔斷裂為巖漿侵入提供了良好通道, 并在巖漿上升過程, 捕獲花崗巖等圍巖, 發(fā)生了地殼混染,形成橄輝云斜煌斑巖。即老廠煌斑巖形成于俯沖-碰撞后的板內(nèi)拉伸構(gòu)造環(huán)境, 可能起源于三疊紀(jì)時(shí)期古特提斯洋板塊俯沖的殘留板片對(duì)地幔楔的交代富集作用形成的源區(qū)[39]。

華南地區(qū)該時(shí)期表現(xiàn)為大規(guī)模的巖石圈伸展(100～80 Ma[38]), 也是滇東南個(gè)舊 Sn-Cu、都龍Sn-W-Zn、白牛廠Sn-Ag礦床, 桂西大廠Sn礦床, 滇黔桂卡林型Au礦床和大瑤山-大明山淺成低溫?zé)嵋盒虯u-Cu礦床形成的動(dòng)力學(xué)背景。因此, 大陸巖石圈地幔伸展作用, 既能形成廣泛的富鉀巖漿巖(鉀質(zhì)-超鉀質(zhì))巖漿活動(dòng), 也與 Cu-Au-Sn多金屬礦床的形成有關(guān)。揚(yáng)子地塊西南緣(哀牢山縫合帶)分布著眾多的與 Cu-Au-Ni礦有關(guān)的漸新世(37～26 Ma)煌斑巖;個(gè)舊老廠煌斑巖與其巖石地球化學(xué)特征相似(圖 1a和表 6)。目前較為統(tǒng)一的觀點(diǎn)認(rèn)為這些煌斑巖起源于古特提斯洋俯沖板片交代的富集地幔(EM2型)[98],因此, 從大地構(gòu)造背景和巖漿起源角度對(duì)比, 個(gè)舊晚白堊世煌斑巖與西南三江漸新世煌斑巖起源可能一致(表6), 均受古特提斯洋俯沖板片交代作用的控制, 只是侵位時(shí)間有差異。

表6 研究區(qū)及其附近煌斑巖成因與特征Table 6 Geological characteristics and genesis of lamprophyres from the study area and its vicinity

6 結(jié) 論

(1) 煌斑巖 2個(gè)鋯石 TIMS U-Pb年齡((87.1±1.3) Ma)為捕獲老卡花崗巖體鋯石的年齡;

(2) 煌斑巖6個(gè)黑云母Ar-Ar年齡平均為72.9 Ma,代表老廠煌斑巖侵位時(shí)代, 老廠煌斑巖屬于個(gè)舊中-酸性巖漿活動(dòng)(85～78 Ma)后基性巖墻群的一部分;

(3) 煌斑巖輝石斑晶和微晶成分近一致, 均屬普通輝石, 黑云母斑晶屬殼源鐵質(zhì)-鎂質(zhì)黑云母, 而黑云母微晶屬鎂質(zhì)黑云母, 中長石斑晶偏基性(An25～40),透長石微晶偏酸性(An1～6);

(4) 老廠煌斑巖形成于俯沖-碰撞后的板內(nèi)拉伸構(gòu)造環(huán)境, 可能起源于三疊紀(jì)時(shí)期古特提斯洋板塊俯沖的殘留板片對(duì)地幔楔的交代富集作用形成的富集地幔源區(qū)。

黑云母Ar-Ar同位素測試和電子探針測試的過程中得到了中國地質(zhì)大學(xué)(北京)王瑜和尹京武老師的無私幫助和指導(dǎo); 兩位審稿專家和編輯對(duì)文章的修改提出了諸多寶貴的意見, 在此一并表示真誠謝意！

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